This invention relates to portal radiography using radiation therapy treatment beams. More particularly, it relates to an assembly useful for radiation oncology portal imaging system using radiographic films and intensifying screens, and to a method of using it.
In conventional medical diagnostic imaging, the object is to obtain an image of a patient""s internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifying screens for imagewise exposure. About 5% or less of the exposing X-radiation passing through the patient is adsorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element.
Examples of radiographic element constructions for medical diagnostic purposes are shown in U.S. Pat. No. 4,425,425 (Abbott et al), U.S. Pat. No. 4,425,426 (Abbott et al), U.S. Pat. No. 4,414,310 (Dickerson), U.S. Pat. No. 4,803,150 (Kelly et al) and U.S. Pat. No. 4,900,652 (Kelly et al), U.S. Pat. No. 5,252,442 (Tsaur et al), and Research Disclosure, Vol. 184, August 1979, publication 18431.
Radiation oncology is the branch of radiology directed to radiation treatment of cancers. Much of the work is called teletherapy, that is the use of powerfill, high-energy X-ray machines (often linear accelerators) to irradiate the cancerous tissues. The goal of the treatment is to cure the patient by selectively irradiating the cancer with sufficiently high dosage to destroy it, yet minimizing the radiation impacting adjacent normal tissues.
Such treatments are commonly made using high energy X-rays (generally 4000 to 25,000 kVp). The X-ray beams are carefully mapped for intensity and energy. The patient is carefully imaged using a conventional diagnostic X-ray machine and/or a CT scanning machine and/or an MRI scanning machine to accurately locate features in the patient""s anatomy. With this information, a dosimetrist determines where and for how long the treatment X-rays should be directed. The dosimetrist uses a computer to predict the radiation dose necessary for the patient""s condition. This may lead to some normal tissues being too greatly exposed. The dosimetrist will then use one or more xe2x80x9cblocksxe2x80x9d or shields to block radiation from reaching the patient""s normal tissues. These xe2x80x9cblocksxe2x80x9d are custom shaped for each patient and are typically made from thick pieces of lead.
Portal radiography is used to provide images to position and confirm radiotherapy in which the patient is given a dose of high energy X-radiation (from 4 to 25 MVp) through a xe2x80x9cportxe2x80x9d in a radiation shield. The object is to line up the port with a targeted tumor so it receives a cell-killing dose of X-radiation. In localization imaging the portal radiographic element is briefly exposed to the X-radiation passing through the patient with the shield removed and then with the shield in place. Exposure without the shield provides a faint image of anatomical features that can be used as orientation references near the target (e.g., tumor) area while the exposure with the shield superimposes a second image of the port area. The exposed localization radiographic element is quickly processed to produce a viewable image and confirm that the port is in fact properly aligned with the intended anatomical target. During the above procedure patient exposure to high energy X-radiation is kept to a minimum. The patient typically receives less than 20 RADs during this procedure.
Thereafter, before the patient is allowed to move, a cell-killing dose of X-radiation is administered through the port. The patient typically receives from 50 to 300 RADs during this step. Since any movement of the patient between the localization exposure and the treatment exposure can defeat the entire alignment procedure, the importance of minimizing the time elapsed during the element processing cycle is apparent. Reducing this time by even a few seconds is highly beneficial.
A second, less common form of portal radiography is the verification of the location of the cell killing exposure. Again, the object is to record enough anatomical information to confirm that the cell killing exposure was properly aligned with the targeted anatomy.
It is appreciated that the large differences in exposure times that distinguish localization and verification imaging have up to the date of this invention precluded the successful use of a single portal radiographic element to serve both applications.
Both localization and verification portal imaging have suffered from very poor image quality. Anatomical features are often faint, barely detectable or even non-detectable. This has severely restricted reliance on portal radiography.
Although excellent radiographic imaging capabilities have been realized in medical diagnostic imaging, there are fundamental differences in the imaging physics that distinguish and render nonanalogous diagnostic and portal radiographic imaging. In diagnostic imaging X-radiation photon energy of up to 140 kVp is in part absorbed within the patient and in part passed through to be absorbed in a fluorescent intensifying screen to generate light that exposes the radiographic element.
In portal imaging the multi-MVp X-radiation in part passes through the patient unabsorbed and is in part absorbed creating a secondary electron emission. A front metal intensifying screen is relied upon to intercept and absorb the secondary electron emission. This lowers minimum density and significantly enhances image sharpness. Image intensification (raising maximum density and contrast) is achieved by absorbing X-radiation and transmitting to the radiographic element electrons that are thereby generated. The much higher capability of the radiographic element to absorb electrons as compared to X-radiation produces image intensification. Besides supplying electrons that are relied upon to expose the radiographic element, the front intensifying screen further contributes to image sharpness by transmitting to a much lesser extent electrons generated by obliquely oriented (that is scattered) X-radiation that it receives.
In addition to the front metal intensifying screen, which is always present, a back metal intensifying screen can be employed to provide an additional source of electrons for radiographic element exposure.
Portal imaging assemblies can be grouped into two categories. In the first category, the assembly includes one or two metal plates and a photographic silver halide film that is designed for direct exposure to X-rays or electrons. Two such films are commercially available from Eastman Kodak Company as KODAK X-RAY Therapy Localization (XTL) Film and KODAK X-RAY Verification (XV) Film. Such direct X-ray exposure assemblies are illustrated in FIG. 1 (described in more detail below). The advantage of such assemblies is that their contrast is very low so that a wide range of exposure conditions provides useful images. However, due to the high-energy radiation used to produce portal images, the subject contrast is also very low. Coupled with the low contrast of the image receptor system, the final image contrast is low and the images are difficult to read with needed accuracy.
The second portal imaging assembly uses a radiographic photographic silver halide film containing fine grain silver halide emulsions, one or two fluorescent (or phosphor) screens (or intensifying panels) and one or two metal plates. One such assembly is illustrated in FIG. 2 (described in more detail below). Although the assembly in FIG. 2 shows only one metal plate, other assemblies can have both front and back metal plates. Because a significant portion of the film""s exposure comes from the light emitted by the fluorescent screens, it is possible to use high contrast films. These assemblies produce significantly higher contrast than direct exposure assemblies while the speed of both assemblies is similar. The resulting images have much higher Noise Equivalent Quanta (xe2x80x9cNEQxe2x80x9d), show clearer definition of imaged features and are much easier to xe2x80x9creadxe2x80x9d than direct exposure images.
Due to the high contrast of both of these types of assemblies and variations in treatment dose, patient anatomy and tissue composition, equipment and other treatment variables, it is difficult to obtain a correct exposure of each patient. The resulting images are either too light or too dark. Exposure is often controlled by adjusting the xe2x80x9cair gapxe2x80x9d distance between the patient and the imaging system. Unfortunately, many therapy machines do not allow for an adjustable xe2x80x9cair gapxe2x80x9d and thus exposure adjustment is limited. This is especially true for verification imaging systems.
Useful portal imaging films and assemblies are described in U.S. Pat. No. 5,871,892 (Dickerson et al).
There is a need for a portal verification imaging assembly and system that can be used on any therapy machine and with which exposure can be suitably adjusted as needed.
This invention provides an assembly for radiation portal imaging comprising in association:
a) a radiographic film comprising a support and a radiation-sensitive layer disposed thereon,
b) an fluorescent intensifying screen, and
c) disposed between the radiographic film and the fluorescent intensifying screen, a filter that has a density of at least 0.05 and attenuates the light emitted from the intensifying screen during exposure of the radiographic film, the filter being disposed on the radiation-sensitive layer of the radiographic film.
This invention also provides a light-tight imaging article comprising a light-tight container that contains the assembly described above.
Further, the present invention provides a method of portal imaging comprising:
A) positioning a patient between a source of imaging radiation and the light-tight imaging article described above, and
B) exposing the imaging article to imaging radiation of from about 4,000 to about 25,000 keV after the radiation passes through the patient.
The radiation portal imaging assembly of the present invention can be used in portal imaging in any suitable therapy machine because it includes one or more filters that attenuates or controls the light emitted from the intensifying screen during exposure to reach the radiographic film. With modest experimentation, it can be readily determined what filter may be useful for a given patient and therapy machine. Thus, the assembly can be easily used to correct for density errors due to patient dose, invariant air gaps, different exposure energies or any combination of these parameters. The assembly is also useful with any portal imaging systems using one or two intensifying screens, with or without accompanying metal screens.